Referring to FIG. 6, there is shown a conventional remote sensor located in an aircraft which may be used for identifying a substance in the atmosphere. As shown, remote sensor 60 includes optical transmitter 62 for transmitting an optical beam through a substance of interest towards the earth (hard target). The optical beam, which is reflected from the earth, passes again through the substance of interest and received by optical receiver 63. A data acquisition system, generally designated as 61, analyzes the returned beam, which may be greatly attenuated by the substance of interest. The data acquisition system makes a determination about the quantity and/or the identity of the substance of interest.
The conventional sensor system 60 may be a differential absorption light detecting and ranging (LIDAR) system, or simply a DIAL system. The DIAL system is an active remote sensing based instrument including two or more pulsed laser transmitters which transmit different colors at separate time intervals to probe the atmosphere below the aircraft. Ground reflected light from these lasers is measured by a receiver including photodiodes.
One wavelength of one of the laser transmitters is tuned to an absorption wavelength of methane (for example) in the mid wave infrared part of the electromagnetic spectrum (known as the online wavelength). One (or more) wavelengths of other laser transmitter(s) are tuned to a wavelength near but not on an absorption wavelength of methane (known as the offline wavelength(s)). An optical receiver measures both the outgoing laser intensities and the backscattered ground return intensities. The online and offline wavelengths are measured by time differences in transmitted and returned spectral content. Concentration measurements of methane are made using algorithm calculations based on these optical measurements.
Another conventional sensor system 60 may be an active remote sensing system which does not use pulsed optical signals, but uses two or more continuous wave (CW) laser sources. The CW signals are combined and transmitted below the airborne platform. Different color CW beams may be combined by an optical combiner, such as a fiber optic or an optical mask, and transmitted downwardly as a single beam. The light in this transmitted beam may include online and offline wavelengths in the near infrared wavelength band, near 1.2-1.5 microns (for example).
These exemplary wavelengths are characteristic of specific carbon dioxide (CO2) and oxygen (O2) absorption features. Ground returns from these transmitted wavelengths may be collected in a fiber optic based receiver, where different color content may be measured by a demodulation technique similar to an FM radio receiver, but designed for near infrared wavelengths. Spectral content of the transmitted and returned beams may be used to determine CO2 concentrations based upon these optical measurements. The O2 channel wavelengths may be used for air temperature and pressure compensation corrections, which are needed during high altitude operation.
Even in a differential technique, such as DIAL, which is designed to reduce noise factors, factors such as atmospheric interference, irregular surface reflectance, object interference (bushes, trees, power line, etc.), source laser misalignment, source laser energy distribution and relative motion between the remote sensing platform and the target may lead to poor data quality. This poor data quality, in turn, may lead to missed detection or false positive detection.
For most DIAL techniques, conventional systems use two lasers (or one laser that produces two wavelengths) in which one laser is designated the online laser and the other laser is designated the offline laser. The online laser signal is designed to be absorbed by the chemical species of interest, while the offline laser signal is designed not to be absorbed by the chemical species of interest. By measuring the transmitted and returned energies for both lasers and applying a differential data processing technique, one may measure the path-integrated concentration, or concentration path-length product (also referred to as the concentration path length (CPL)) of the chemical species in the column of air to a particular target location. This differential measurement helps reduce noise factors but the measurement may further be refined by allowing a longer sample exposure or by integrating samples of the same column of air. However, this tends to defeat one of the advantages of a remote sensing system which is rapid area coverage.
In general, a DIAL system, which uses pulsed optical signals and attempts to measure the concentration of CH4 in a column of air, may have a limited background measurement accuracy (for example, approximately 50 ppm*m (parts per million*meter), or as another example, approximately +/−15% of the background level) The DIAL system is typically designed to locate plumes only and not calculate accurate background CH4 levels. An upper range for plume quantification, for example, may be about 4000 ppm*m with reduced accuracy. The coverage rate, while flying in an aircraft, may be moderate and may typically be limited to 1.8 miles2/hr, as an example. The DIAL system also may have limited detection capabilities over water.
In the other sensor system, which is described above as a CW optical system attempting to measure and quantify CO2 concentrations in the atmosphere, there are also several deficiencies. There is limited plume location and quantification capability. Instrument modality requires fairly long temporal integration, due to speckle noise surrounding extremely narrow spectral content. This leads to poor ground spot resolution (GSR) for the gas measurements. Ground spot resolution is typically on the order of square miles, which limits surveys over small targets, or regions of interest.
In order to identify and quantify two types of measurements for two gasses (for example, methane and carbon dioxide), conventional active remote sensor systems would require two types of DIAL sensors and two types of CW optical systems. That is, two types of DIAL sensors would be required to provide plume quantification and background measurements for methane; and two types of CW optical sensors would be required to provide plume quantification and background measurements for carbon dioxide. This potential need for 4 sensors on one platform is considered a major drawback, if DIAL modalities are to be used for measuring greenhouse gas sources of both CO2 and CH4 (for example).
As will be explained, the present invention provides a system and method for improving the quality of detection and quantification of one or multiple species of gas in a column of air, when examined from an airborne platform, such as an aircraft or satellite. The platform may, of course, also be a van or a truck. In addition, the present invention provides a new type of DIAL modality using light source(s) which have sufficient spectral content to probe, or investigate the atmospheric species of gas.
As will also be explained, the present invention uses mode-locked based laser transmitters to provide stable and simultaneous broad spectral content. For example, four spectral channels may be generated simultaneously by the present invention within one single optical beam. These spectral channels are sub-bands of colors, which are resolved by a grating based receiver and measured simultaneously.